#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    ll n;
    cin >> n;
    ll a[n];
    rep(i,n)cin >> a[i];

    ll ans = 0;
    ll now = 0;

    rep(i,n){
        while(a[i] > 1){
            if(a[now] == 0){
                ans += abs((ll)i - now);
                a[now] = 1;
                a[i]--;
                now++;
            }else{
                now++;
            }
        }
    }

    cout << ans << endl;
    return 0;
}